This invention relates to electronic communication systems and more particularly to wireless communication systems.
Since the introduction of wireless telecommunication systems, the number of mobile users has grown, and is expected to continue growing substantially, especially with mass-market uptake of mobile triple play (a combination of mobile telephony, mobile broadband, and mobile television (TV)). That increase and increasing user demand for higher data rates have created a need for additional frequency bands and user equipment, such as mobile phones and other remote terminals, that supports multiple frequency bands.
Mobile communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as the universal mobile telecommunications system (UMTS), which is a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute within the International Telecommunication Union's IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates specifications for the UMTS and WCDMA systems.
3G mobile communication systems based on WCDMA as the radio access technology (RAT) are being deployed all over the world. High-speed downlink packet access (HSDPA) is an evolution of WCDMA that provides higher bit rates by using higher order modulation, multiple spreading codes, and downlink-channel feedback information. Another evolution of WCDMA is Enhanced Uplink (EUL), or High-Speed Uplink Packet Access (HSUPA), that enables high-rate packet data to be sent in the reverse, or uplink, direction. New RATs are being considered for evolved-3G and fourth generation (4G) communication systems, although the structure of and functions carried out in such systems will generally be similar to those of earlier systems.
WCDMA communication systems currently operate in frequency bands around 850 megahertz (MHz), 1700 MHz (in Japan and the U.S.), 1800 MHz, and 2100 MHz (in the U.S). To enhance capacity and coverage potential in the future, WCDMA systems are expanding to frequency bands around 900 MHz and 2500 MHz. FIG. 1 is a plot of band identification number (on the vertical axis) against frequency (on the horizontal axis) for several WCDMA frequency bands. Details of this arrangement are described in, for example, Section 5 of 3GPP Technical Specification (TS) 25.101 V7.7.0, User Equipment (UE) Radio Transmission and Reception (FDD) (Release 7) March 2007. It will be seen from FIG. 1 that some of the frequency bands overlap, e.g., Bands V, VI, and VIII from 869-915 MHz; and Bands III, IV, and IX at 1710-1785 MHz.
As a result, a UE supporting several frequency bands has to cope with the problem of searching for cells/services in the correct frequency band, which depends on the geographical area that the UE is in. A cell belongs to a public land mobile network (PLMN), and cell/PLMN selection has a number of objectives, which include connecting a UE to the cell(s)/PLMN(s) that will provide the highest quality of service (QoS), enable the UE to consume the least power, and/or generate the least interference. Cell/PLMN selection is usually based on the signal strength (signal to interference ratio (SIR) or signal to noise ratio (SNR)) of candidate cells. For 3GPP-compliant mobile communication systems, the PLMN selection process is specified in Section 4.4 of 3GPP Technical Specification (TS) 23.122, Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode (Release 7), V7.5.0 (June 2006).
When a conventional UE is powered on or has lost service, the UE usually assumes that it is in the same geographical area as it was when it was last powered off or lost service. This is done as a way to optimize cell searching procedures. Thus, a search for a cell/service is started in the last known frequency/frequencies where service was available. If such a search proves fruitless, the typical UE starts an “initial cell search” procedure that involves scanning all RF carriers in the frequency band(s) that the UE believes is or are available in order to find a suitable cell of the selected operator, or PLMN. On each of the RF carriers, the UE searches at least for the strongest cell.
For an example of the current typical operation, assume that a UE capable of handling the WCDMA 2100 MHz frequency band (i.e., Band I in FIG. 1) is turned off in a geographical area (e.g., a country such as Sweden) where the 2100 MHz band actually is used for WCDMA. Assume also that the UE was camped on a cell and service was available before the UE was powered off. When the UE is powered on again, the UE assumes that it has not moved geographically and hence it tries to find the last cell or another cell with the last-known carrier in the 2100 MHz band. If the UE has moved or for some other reason cannot find a cell in the 2100 MHz band, the UE proceeds to scan the 2100 MHz band, measuring its received power on each possible carrier in the band.
The scan procedure, which may be called a received signal strength indicator (RSSI) scan, results in measurements within the relevant channel bandwidth (e.g., 5 MHz) on roughly 300 possible carriers in the 2100 MHz band. An RSSI scan can usually be fast; e.g., it may take about 300 milliseconds (ms) for the UE to scan 300 carriers in the 2100 MHz band. FIG. 2 shows an example of a result of an RSSI scan as a plot of received energy versus frequency, showing energy peaks measured by a UE in the 2100 MHz band. At this stage, no intelligent processing of the received energy has been performed, i.e., the received energy could be anything from noise to cell transmissions.
The typical UE deeply explores (i.e., performs cell search on) each of the frequencies having more than a threshold energy, normally starting around the highest-energy frequencies and working through the rest of the frequencies until a WCDMA cell is found to camp on. Cell search is a time- and energy-consuming procedure for a UE; for example, each cell search may take up to 400 ms. For more efficient search, some UEs include information on how PLMNs are usually planned and start their cell searches based on that information, although this can be a drawback if a network operator re-plans the way the carriers are distributed.
Cell search is traditionally based on the signal strength or SNR of candidate cells. For example, U.S. patent application Ser. No. 11/289,001 filed on Nov. 29, 2005, by B. Lindoff for “Cell Selection in High-Speed Downlink Packet Access Communication Systems”, which is incorporated here by reference, describes a cell selection process that also takes into account the delay spread of the communication channel.
To illustrate some of the problems with existing cell search approaches, assume that a UE supports Bands I, III, and V depicted in FIG. 1, that the UE was camped on a cell in Band I just prior to its being powered off, and that the UE has been moved to a geographical area where Band III is used for WCDMA. With a conventional cell search algorithm, the UE assumes when it is powered on that it is still in the same geographic area. After unsuccessfully searching for a cell on the last-camped-on frequency, the UE performs an RSSI scan in the downlink part of Band I and then conducts a futile search for cells in Band I before it eventually understands that there are no cells available in this band. Much energy and time is wasted on the search for non-existent Band I cells, and even if the UE eventually determines that Band I is not the correct band, the UE does not know which of its other supported bands (Band III and Band V in this example) is correct. Thus, the UE could perform another futile search.
As another example, assume that a UE operating in Band I suddenly finds itself in a radio shadow (e.g., the UE is taken into a basement or is driven into a tunnel), resulting in loss of service. After a long-enough period in the radio shadow, the UE runs an RSSI scan of Band I and determines that no cells are available. The UE may then search the other two bands it supports (Bands III and V in this example), wasting energy and time. If during the time that the UE is searching for cells in the other two bands the radio environment improves (e.g., the UE leaves the basement or tunnel), the UE may not notice as it is busy with the other bands and give the user no service until the UE finds service again in Band I. Of course, such operation is not be well received by the user.
Searching in an incorrect frequency band wastes a substantial amount of electric power, which is a concern for a battery-powered UE, and subjects the user to a substantial amount of time without service. A UE may even falsely believe that energy received from other sources is received from candidate cells (radio base stations (RBSs)), and hence be tricked into searching for cells in vain. This is especially likely in cases where frequency bands overlap each other (see, e.g., Bands I and II around 1900 MHz in FIG. 1). Hence, it is very important for a multi-band UE to use intelligent searching strategies.
European Patent Application EP 1 367 844 A1 and U.S. Patent Application Publication No. U.S. 2003/0236079 describe a cellular phone that includes an RSSI measurement circuit for measuring power levels of received baseband signals at divided band portions of a whole frequency band, a band sorting circuit that sorts the divided band portions based on the descending order of the power levels, and a cell search circuit that searches the carriers of each divided band portion in the order of the sorting results, to thereby determine a tentative waiting cell.
U.S. patent application Ser. No. 11/615,162 by Joachim Ramkull et al. for “Efficient PLMN Search Order” describes how a UE can shorten the time needed to find a cell, such as a suitable or acceptable cell, by using intelligent search orders.